Added blocks to support customization to:
- the logo in the upper right. Both tmpnb and notebooks.azureml.net use
this area to add logos.
- help links. We add links to the help tab. We also add a
rel="noreferrer" to each link. These block enable us to make these
corrections without altering the primary template.
- provide a place in tree to give an attribute area. We use this to
hide the clusters tab.
Min RK
Matthias Bussonnier
Doug Blank
jdfreder
Christopher Wilcox